A tractor beam reels in objects with sound

Ultrasound machine pulls on a target that is large enough to see

BEAM ME IN At left, waves (yellow dashed lines) from a regular beam of light or sound tend to bounce off an object (blue arrows) and deliver a subtle push. Waves from a new tractor beam, illustrated at right, bounce off the sides of an object and rebound upward. As a result, the object gets pulled toward the source of the beam.

Tractor beams have hit the big time. A newly constructed device generates a beam of concentrated sound that, for the first time, exerts a continuous, perceptible tug on objects large enough to see. The researchers didn’t actually reel in an object, but they demonstrated that an ultrasound tractor beam could do the job.

Using a tractor beam to haul a damaged spaceship may look simple on Star Trek, but nature makes it very difficult to pull objects from a distance. Waves of light or sound fired at an object tend to bounce off its surface like raindrops on an umbrella, collectively exerting a subtle nudge called radiation pressure that pushes the object away.

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